Checkered light-intensity pixel data is acquired by shifting the sampling timing of even-numbered horizontal scan lines from the sampling timing of odd-numbered horizontal scan lines by one-half of a sampling cycle. Each of the interpolated light-intensity pixel data for the checkered light-intensity pixel data is obtained by calculating the average value of four checkered light-intensity pixel data on up-and-down and right-and-left sides of each of the interpolated light-intensity pixel data. Light-intensity image data is generated by interpolating the interpolated light-intensity pixel data into the checkered light-intensity pixel data. The data interval in the light-intensity image data becomes twice the pixel density of the checkered light-intensity pixel data.